Photoelectrophoretic reversal imaging using a suspension containing a vitamin precursor, {62 -carotene

ABSTRACT

There is disclosed a photoelectrophoretic imaging system wherein the imaging suspension utilized contains a photomigratory pigment dispersed in an insulating carrier liquid. The imaging suspension contains a vitamin precursor, beta carotene, in conjunction with the other components of the vehicle. The suspension is interpositioned between at least two electrodes and subjected to a potential difference while substantially simultaneously being selectively exposed to a reproducible image by a source of electromagnetic radiation. As a result of the suspension of the present invention it is now possible by regulating the polarities of the electrodes in the system to produce both negative and positive images from a single sense input.

United States Patent Jelfo May 23, 1972 [54] PHOTOELECTROPHORETIC3,384,566 5/1968 Clark ..204/ 181 REVERSAL IMAGING USING A SUSPENSIONCONTAINING A VITAMIN OTHER PUBL'CAT'ONS PRECURSOR B CAROTENE Cherry etal., Photoconductivity of Carotenoids," 1967,

' Vol. 3, Molecular Crystals, pp. 251-259 [72] Inventor: Raymond L.Jelfo, Sodus Point, NY.

Primary Examiner-George F. Lesmes [73] Assignee. Xerox Corporation,Rochester, NY. Assistant Examiner R E Martin [22] Filed: Dec. 24, 1969Attorney-James J. Ralabate, Donald C. Kolasch and Albert 21 Appl. No.:887,879 Mahasse' Related U.S. Application Data Continuation-in-part ofSer. No. 642,005, May 29, 1967, abandoned.

References Cited UNITED STATES PATENTS Tulagin et al. ..252/501 XTulagin et al. ..96/1.3 X

[57] ABSTRACT There is disclosed a photoelectrophoretic imaging systemwherein the imaging suspension utilized contains a photomigratorypigment dispersed in an insulating carrier liquid. The imagingsuspension contains a vitamin precursor, beta carotene, in conjunctionwith the other components of the vehicle. The suspension isinterpositioned between at least two electrodes and subjected to apotential difference while substantially simultaneously beingselectively exposed to a reproducible image by a source ofelectromagnetic radiation. As a result of the suspension of the presentinvention it is now possible by regulating the polarities of theelectrodes in the system to produce both negative and positive imagesfrom a single sense input.

4 Claims, 1 Drawing Figure PHOTOELECTROPHORETIC REVERSAL IMAGING USING ASUSPENSION CONTAINING A VITAMIN PRECURSOR, B-CAROTENE This applicationis a continuation-in-part of U.S. Pat. application Ser. No. 642,005filed May 29, 1967, now abandoned, and relates to an imaging system andmore specifically to a photoelectrophoretic imaging system.

BACKGROUND OF THE INVENTION In photoelectrophoretic imaging coloredphotosensitive particles are suspended in an insulating carrier liquid.This suspension is then placed between at least two electrodes,subjected to a potential difference and exposed to a light image.Ordinarily, in carrying out the process, the suspension is placed on atransparent electrically conductive member in the form of a thin film,and exposure is made through this member while a second generallycylindrically shaped biased electrode is rolled across the suspension.The particles are believed to bear an initial charge when suspended inthe liquid which causes them to be attracted to the transparent baseelectrode and upon exposure, to change polarity so that the exposedparticles migrate to the second or roller electrode thereby formingcomplementary images on each of the electrodes by particle subtraction.The process may be used to produce both polychromatic and monochromaticimages. In the latter instance a single color photoresponsive particlemay be used in the suspension or a number of differently coloredparticles may be used all of which respond to the exposure radiation. Anextensive and detailed description of the photoelectrophoretic imagingtechniques as described above may be found in U.S. Pat. Nos. 3,383,993;3,384,488; 3,384,564; and 3,384,566, and are hereby incorporated byreference.

Although it has been found that good quality images can be produced,especially when a relatively insulating blocking electrode surface isused, due to the nature of the process the photoresponsive particlestend to respond in one particular imaging mode. Thus, in a monochromaticsystem, acceptable imaging has been found to be generally restricted toa single sense process so that with positive input a negative image maybe reproduced. In order to obtain a positive output the image input mustbe a negative image.

It is, therefore, an object of this invention to provide an imagingsystem which will overcome the above noted disadvantages.

It is a further object of this invention to provide a novel imagingsystem capable of a high degree of flexibility.

Another object of this invention is to provide a novel imaging systemcapable of producing high contrast, background free images in a one stepprocess.

Yet, still a further object of this invention is to provide a novelphotoelectrophoretic monochromatic imaging system.

Still a further object of the present invention is to provide a novelmonochromatic imaging composition.

Yet, still another object of the present invention is to provide a onestep imaging system capable of producing high contrast images of eitheroptical sense regardless of the sense of the input information.

SUMMARY OF THE INVENTION The foregoing objects and others areaccomplished in accordance with this invention, generally speaking, byproviding a suspension of photoelectrophoretic imaging particles in aninsulating carrier liquid. The suspension utilized in the process of thepresent invention contains B Carotene, a vitamin precursor additive,which makes it possible upon proper orientation of the electrodepolarities to readily produce high contrast, background free images inboth a positive and negative imaging sense. The suspension isinterpositioned between at least two electrodes and subjected to anelectrical field. The suspension is selectively exposed to areproducible image by a source of electromagnetic radiation. Generallyspeaking, the imaging suspension is placed on a transparent electricallyconductive member or first electrode in the form of a thin film, andexposure is made through the transparent member while in contact with asecond imaging electrode which is placed or rolled over the top of theimaging suspension. The photomigratory particles present in thissuspension respond to the electromagnetic radiation to form a visibleimage pattern at one or both of the electrodes, the images beingcomplementary in nature. The imaging suspension employs intenselycolored pigment'particles the photosensitive pigment portion of whichserves both as the colorant and as the photosensitive material.Additional photosensitive materials are not required thus providing avery expedient imaging process.

It has been determined that upon introducing [3 Carotene, a vitaminprecursor, into the photomigratory imaging suspension of the presentinvention it is possible, upon proper orientation of the electrodepolarities, to produce high contrast, background free images in either apositive to negative or positive to positive imaging mode. Heretonow,the materials used as components of the-imaging suspension havesubstantially restricted 'the imaging capabilities of thephotoelectrophoretic monochromatic imaging'system. It has now beendemonstrated that upon introducinginto the imaging suspension the [3Carotene additive, it is now possible to produce images of either senseby changing the polarity of the bias on the electrodes in the system.

DESCRIPTION OF THE DRAWINGS The invention is more specifically definedin the accompanying drawing in which there is seen a transparentelectrode generally designated 1 which, in this instance, is made up ofa layer of optically transparent glass 2 overcoated with a thinoptically transparent layer 3 of tin oxide. Tin oxide coated glass ofthis nature is commercially available under the tradename NESA glass.This electrode shall hereafter be referred to as the injectingelectrode. Coated on the surface of the injecting electrode 1, is a thinlayer 4 of finely divided photosensitive pigment particles dispersed inan insulating carrier liquid. The term photosensitive," for the purposesof this application, refers to the properties of a particle which, onceattracted to the injecting electrode, will migrate away from thiselectrode under the influence of an applied electric field when it isexposed to actinic radiation. A further detailed explanation of theapparent mechanism of the operation is disclosed in the above U.S. Pat.Nos. 3,384,565 and 3,384,566.

The imaging suspension will consist of specifically colored, finelydivided photosensitive particles dispersed in an insulating carrierliquid or vehicle. Any suitable photosensitive pigment particle may beused such as disclosed in U.S. Pat. Nos. 3,384,565 and 3,384,566. Asabove stated, the pigment portion of the photomigratory particleprovides both the photosensitivity and coloration for the respectiveparticle. Any suitable insulating carrier liquid may be used in thecourse of the present invention. Typical insulating carrier liquidsinclude long chain saturated aliphatic hydrocarbons such as decane,dodecane and tetradecane, kerosene fractions such as Sohio OdorlessSolvents available from Standard Oil Company of Ohio, Isopar Gcommercially available from Humble Oil Company of New Jersey andparaffin wax, molten beeswax and other molten thermoplastic materials,mineral oil, corn oil, linseed oil, olive oil, marine oils such as spermoil and cod liver oil, silicone oils such as dimethyl polysiloxane (DowCorning Co.), fluorinated hydrocarbons such as Freon and mixturesthereof. The imaging suspension may also contain a sensitizer and/orbinder for the pigment particles. To the imaging suspension of thepresent invention is added a vitamin precursor ingredient, betacarotene, generally in amounts ranging from about 0.0025 to about 1.0gram per gram of photoresponsive or photomigratory pigment. Preferredamounts fall in the range of from about 0.0125 to 0.5 grams with optimumresults being obtained in a range of from about 0.02 to about 0.1 gram.

Above the liquid suspension is passed a second or imaging electrode 5which, in this illustration is represented as a roller having aconductive central core 11 connected to a power source 6. The core inthis instance is covered with a layer 12 of blocking material which may,for example, be polyurethane, capable of blocking DC current andreferred to as a blocking layer. A transfer sheet 13 of ordinary bondpaper is attached to the outer surface of the roller. The blocking orimaging electrode is connected to one side of potential source 6 throughswitch 7. The opposite side of potential source 6 is connected to theinjecting electrode 1 so that when switch 7 is closed an electric fieldis applied across the liquid suspension 4 between electrodes 1 and 5.The pigment suspension is exposed by way of the projector mechanism madeup of a light source 8, a transparency 9, and a lens system 10. Forpurposes of this illustration a microfilm positive is used during theprocess. A detailed description of the types of materials which may beemployed as the blocking layer may be found in U.S. Pat. No. 3,383,993.The blocking electrode 5, having a cylindrical configuration in thepresent illustration, is rolled across the top surface of the injectingelectrode 1 supporting the suspension 4 containing the photomigratoryparticles. Switch 7 is closed during the period of image exposure. Uponproper orientation of the electrode polarities, the light exposurecauses the exposed pigment particles suspended in the carrier to migrateto the surface of electrode 5 leaving behind a complementary image onthe surface of the injecting electrode 1 of the unexposed particles.Upon reversing the polarities at which the field is applied the imagesense is reversed while maintaining the same input information. Itshould be noted at this point of the discussion that although theblocking electrode in the present illustration is represented as acylinder it may also take the form of a flat plate electrode, as in thecase of the illustrated injecting electrode, and the blocking electrodecould be the optically transparent electrode and exposure made throughit. Thus, it is to be understood that it is not intended that thestructural arrangement of the apparatus represented by the illustrationbe restricted to the design as set out herein and all similarconfigurations which will satisfy the requirements of the presentinvention are contemplated. For example, all electrodes utilized may becylindrically shaped thus providing for an expedient continuous process.

The pigment image produced need not necessarily be formed on the surfaceof an electrode but may in fact be formed on a removable paper substrateor sleeve superimposed on or wrapped around the blocking electrode orotherwise interpositioned between the electrodes at the site of imaging.The pigment image may then be fixed in place as for example by placing alamination over its top surface such as by spraying with a thermoplasticcomposition, or by solvent evaporation. The image may also betransferred to the surface of a receiver substrate to which it may inturn be fixed. This would especially be desirable in the case where theimage is formed directly on the electrode surface. Such a transfer stepmay be carried out by adhesive pickoff techniques or preferably byelectrostatic field transfer while the image is still wet. The blockinglayer itself may be in the form of a removable sleeve in which instanceit is simply replaced following imaging with a similar material. Whenthe image is formed on a substrate wrapped about or superimposed on theelectrode itself it is only necessary to disengage the substrate fromthe electrode surface. The system herein described produces a highcontrast monochromatic image with little or no background degradation ineither a positive or negative or positive to positive imaging mode by asimple reversing of the polarities within the system.

Any suitable material may be used as the receiving or transfer substratefor the image produced such as paper or various transparent plasticssuch as Mylar (polyethylene terephthalate), Tedlar (polyvinylfluoride)or cellulose acetate sheets, the latter particularly if it is desirableto produce a transparency suitable for image projection.

When used in the course of the present invention, the term injectingelectrode should be understood to mean that it is an electrode whichwill preferably be capable of exchanging charge with the photosensitiveparticles of the imaging suspension when the suspension is exposed tolight so as to allow for a net change in the charge polarity on theparticle. By the term blocking electrode is meant one which issubstantially incapable of injecting charge carriers into the abovementioned photosensitive particles thus substantially blocking D.C.current. The use of the blocking electrode serves to minimize particleoscillation in the system.

It is preferred that the injecting electrode be composed of an opticallytransparent material, such as glass, overcoated with a conductivematerial such as tin oxide, copper, copper iodide, gold or the like;however, other suitable materials including many semiconductivematerials such as raw cellophane, which are ordinarily not thought of asbeing conductors but which are still capable of accepting injectedcharge carriers of the proper polarity under the influence of an appliedelectric field may be used within the course of the present invention.The use of more conductive materials allows for cleaner chargeseparation and prevents possible charge buildup on the respectiveelectrode, the latter tending to diminish the interior electrode field.The blocking layer of the imaging electrode, on the other hand, isselected so as to prevent or greatly retard the injection of chargecarriers into the photosensitive pigment particles when the particlesreach the surface of this electrode. Although a blocking electrodematerial need not necessarily be used in the system, the use of such alayer is preferred because of the markedly improved results which it iscapable of producing. It is preferred that the blocking layer, whenused, be either an insulator or a semiconductor which will not allow forthe passage of sufficient charge carriers, under the influence of theapplied field, to discharge the particles finely bound to its surfacethereby preventing particle oscillation in the system. The result isenhanced image density and resolution. Even if the blocking layer doesallow for the passage of some charge carriers to the photosensitiveparticles it still will be considered to fall within the class ofpreferred materials if it does not allow for the passage of sufficientcharge so as to recharge the particles to the opposite polarity.Exemplary of the preferred blocking materials used are baryta paper,Tedlar, Mylar and polyurethane. Any other suitable materials having aresistivity of from about 10 ohmscm. or greater may be employed. Typicalmaterials in this resistivity range include cellulose acetate coatedpapers, cellophane, polystyrene and polytetrafluoroethylene. The core ofthe blocking electrode generally will consist of a material which isfairly high in electrical conductivity. Typical conductive materialsincluding conductive rubber, and metal foils of steel, aluminum, copperand brass have been found suitable. Preferably, the core of theelectrode will have a high electrical conductivity in order to establishthe required field differential in the system; however, if a materialhaving a low conductivity is used a separate electrical connection maybe made to the back of the blocking layer of the blocking electrode. Forexample, the blocking layer or sleeve may be semiconductive polyurethanematerial having a conductivity of from about 10 to 10' ohms-cm. If ahard rubber non-conductive core is used then a metal foil may beemployed as a backing for the blocking sleeve. Other materials that maybe used in conjunction with the injecting and blocking electrodes andother photosensitive particles which may be used as the photomigratorypigments and the various conditions under which the process operates maybe found in the above cited issued patents U.S. Pat. Nos. 3,384,565 and3,384,566 as well as U.S. Pat. Nos. 3,384,488 and 3,383,993.

It is to be understood that any suitable photosensitive pigment particlesuch as identified in the above cited patents may be employed within thecourse of the present invention with the selection depending largelyupon the photosensitivity and the spectral sensitivity desired. Typicalphotoresponsive organic materials include substituted and unsubstitutedorganic pigments such as phthalocyanines, for example, copperphthalocyanine; beta form of metal-free phthalocyanine;tetrachlorophthalocyanine; and x-form of metal-free phthalocyanine;quinacridones as for example 2,9-dimethyl quinacridone; 4,l1-dimethylquinacridone; 3,l0-dichloro-6,l3- dihydro-quinacridone; 2,9-dimethoxy-6,l B-dihydro-quinacridone and 2,4,9,l l-tetrachloro-quinacridone;anthraquinones such as l,5-bis-(betaphenylethylamino) anthraquinone;l,5-bis-(3-methoxypropylamino) anthraquinone; l,2,5,6-di(C,C'-diphenyl)-thiazole-anthraquinone; 4-(2hydroxyphenyl-methoxyamino)anthraquinone; triazines such as 2,4- diaminotriazine; 2,4-di-( l'-anthraquinonyl-amino)-6-( 1"- pyrenyl)-tn'azine; 32,4,6tri-(1,1",l"-pyrenyl)-triazine; azo compounds such as 2,4,4-tris(N-ethyl-N hydroxy-ethyl-paminophenylazo) phloroglucinol;l,3,5,7-tetrahydroxy- 2,4,6,8-tetra(N-methyl-N-hydroxy-ethyl-p-amino-phenylazo) naphthalene;l,3,5-tri-hydroxy-2,4,6-tri (3-nitro-N-methyl-N-hydroxy-methyl-4'-aminophenylazo) benzene; metal salts and lakes ofazo dyes such as calcium lake of 6-bromo-1 (I'- sulfo-2-naphthylazo)-Z-naphthol; barium salt of 6-cyano-l (l-sulfo-2-Naphthylazo)-2-naphthol;calcium lake of l'(2'- azonaphthalienel -sulfonic acid)-2-naphthol;calcium lake of l-( 4'5 -chloroazo-benzene-2-sulfonic acid)-2-hydroxy-3naphthoic acid; and mixtures thereof. Other organic pigments includepolyvinylcarbazole; tri-sodium salt of 2-carboxyl phenyl azo(2-naphthiol-3,6-disulfonic acid; N-isopropylcarbazole; 3-benzylideneaminocarbazole; 3-aminocarbazole; l-(4-methyl-5 -chloro-2-sulfonic acid)azobenzene-Z- hydroxy-S-naphthoic acid; N-2 pyridyl-8, l3-dioxodinaphtho-(2,l-b; 2' ,3-d)-furan-6-carboxamide; 2-amino-S-chloro-p-toluene sulfonic acid and the like.

Typical inorganic photosensitive compositions include cadmium sulfide,cadmium selenide, cadmium sulfoselenide, zinc oxide, zinc sulfide,sulfur, selenium, antimony sulfide, lead oxide, lead sulfide, arsenicsulfide, arsenic-selenium, sulfurselenium, and mixtures thereof. Theimaging suspension may contain one or more different photosensitiveparticles each having various ranges of spectral response.

A wide range of voltages may be applied between the electrodes in thesystem. For good image resolution, high image density and low backgroundit is preferred that the potential applied to such as to create anelectric field of at least about 300 volts per mil across the imagingsuspension. For example, when the imaging suspension is coated to athickness of about 1 mil the electrode spacing will be such that anapplied potential of about 300 volts produces a field across thesuspension of about 300 volts per mil. Potentials as high as 8,000 voltshave been applied to produce images of high quality. As is apparent theapplied potential necessary to obtain the desired field of strength willvary depending upon the interelectrode gap as well as the type andthickness of the blocking material utilized. The upper limit of thefield strength appears to be limited primarily by the breakdownpotential of the suspension. The imaging suspension is generally coatedto a thickness of up to about 1 mil or 25 microns, with a preferredoperational thickness being in the range of from about 3-5 microns.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To further define the specificsof the present invention, the following examples are intended toillustrate but not limit the particulars of the present system. Partsand percentages are by weight unless otherwise indicated.

In the following examples, the injecting first electrode consists ofNESA glass as described above. The imaging or blocking electrodeconsists of a conductive steel core coated with a layer of polyurethane,unless otherwise indicated. A potential of about 7,000 volts is appliedacross the imaging suspension.

EXAMPLE I A commercial, metal-free phthalocyanine is purified by acetoneextraction to remove organic impurities. Since this extraction stepyields the less sensitive beta crystalline form, the desired alpha fonnis obtained by dissolving grams of the beta form in 600 cc. of sulfuricacid, precipitating it by pouring the solution into 3,000 cc. of icewater and washing with water to neutrality. The thus purified alphaphthalocyanine is then salt milled for six days and desalted byslurrying in distilled water, vacuum filtering, water washing, andfinally, methanol washing until the initial filtrate is clear, thus,producing x-form phthalocyanine. After vacuum drying to remove residualmethanol, the x-form phthalocyanine produced is used to prepare animaging suspension according to the following formulation:

phthalocyanine (x-form) 10 grams beta carotene (,B) .3 grams sperm oil(ADM 38 BW) 250 cc.

tricresyl phosphate 18 grams The phthalocyanine is ground in a mortar,placed in a Waring blender with the other ingredients and dispersed forabout 10 minutes at high speed. The resulting suspension is coated onthe surface of a NESA glass electrode. As the blocking electrode withpaper sleeve is passed across the surface of the suspension liquid at arate of about 4 inches per second, the potential is established asstated above and the suspension is selectively exposed to a lightintensity of about 12 foot-candles through a positive transparency witha General Electric visible light source. The blocking electrode ismaintained as the positive pole and the NESA glass electrode thenegative pole. By a single material transfer a high quality negativeimage with little or no background is formed on the surface of the papersleeve.

EXAMPLE II The process of Example I is repeated with the exception thatthe polarities on the blocking electrode and the NESA glass electrodeare reversed. There results a high quality positive image on the surfaceof the paper sleeve thereby demonstrating the capability of the presentsystem to image in a direct manner in a positive to positive imagingmode. Comparing the results of Example I and Example 11, there isdemonstrated the claimed capabilities of the present invention, that is,the capability of imaging in either a positive to negative or positiveto positive imaging mode, obtaining images in a single pass system ofhigh quality, high contrast, low background characteristics.

EXAMPLE III An imaging suspension of the following formulation isprepared:

phthalocyanine (x-form) 4 grams olive oil 20 cc. mineral oil 56 cc.tricresyl phosphate 4 grams beta carotene 0.l gram The phthalocyanine,prepared according to the process of Example I, is ground in a mortar,placed in a Waring blender with the other ingredients and dispersed forabout 10 minutes at high speed. The resulting suspension is coated onthe surface of the NESA glass electrode and imaged through a positivetransparency as in Example I with the blocking electrode with papersleeve having a positive polarity and the NESA glass injecting electrodea negative polarity. The resulting image produced on the paper sleeve issimilar to that obtained in Example 1.

EXAMPLE IV The process of Example III is repeated with the exceptionthat the polarities on the two electrodes are reversed. The resultinghigh quality positive image obtained on the paper sleeve furtherdemonstrates the high quality positive to positive image capable of thepresent system.

EXAMPLES V & VI

Two separate imaging suspensions are prepared according to the processof Example I. However, in the present examples, the beta carotenecomponent is excluded from each formulation. The imaging suspensionprepared is coated on the NESA electrode. The polarity of the blockingelectrode is made positive and the polarity on the injecting electrodenegative. The resulting image produced on the paper sleeve of Example Vaccording to the process described in Example I is of a high quality,low background nature. Next, the polarities of the two electrodes arereversed with the blocking electrode made negative and the injectingelectrode positive. The resulting image produced from this arrangement,Example V], is of a low quality, low contrast nature. In comparing thesetwo examples, that is, Examples V and VI, it is evident that theelimination of the beta carotene component from the imaging formulationrestricts the image capability of the system, to a positive to negativeor negative to positive imaging system.

EXAMPLE VII The process of Example I is repeated with the exception thatthe following formulation is utilized:

Watchung Red B 1 gram phthaiocyanine (x-form) 4 grams mineral oil 80 cc.tricresyl phosphate grams beta carotene 0.05 grams Utilizing a positivetransparency at the input end with the blocking electrode having anegative polarity and the injecting electrode a positive polarity a highquality positive image is produced on the paper sleeve of the blockingelectrode.

EXAMPLE VIII The process of Example I is repeated with the exceptionthat the following formulation is utilized:

Algol Yellow l gram phthalocyanine (x-form) 4 grams tricresyl phosphate2 grams linseed oil 106 cc. styrene 20 grams beta carotene 0.05 gramsWith a negative transparency at the input end and with the blockingelectrode having a negative polarity and the injecting electrode apositive polarity, a high quality negative image is produced on thesurface of the paper sleeve of the blocking electrode.

Although the present examples were specific in terms of conditions andmaterials used, any of the above listed typical materials may besubstituted when suitable in the above exam ples with similar results.In addition to the steps used to carry out the process of the presentinvention, other steps or modifications may be used, if desirable. Forexample, the process may be readily adapted to be used in a continuousimaging system. In addition, other materials may be incorporated in theimaging suspension, injecting electrode or blocking electrode toenhance, synergize or otherwise desirably effect the properties of thissystem for their present use. For example, the imaging suspension maycontain sensitizers for the photoconduetive particles which aredissolved or suspended in the carrier liquid.

Anyone skilled in the art will have other modifications occur to himbased on the teachings of the present invention. These modifications areintended to be encompassed within the scope of this invention.

What is claimed is:

l. A photoelectrophoretic imaging process comprising subjecting a layerof an imaging suspension to an applied electric field between at leasttwo electrodes at least one of which is partially transparent, saidsuspension comprising a plurality of photoelectrophoretic imaginparticles and a vitamin precursor, beta carotene, disperse in aninsulating carrier liquid, each of said particles comprising anelectrically photosensitive pigment, said pigment being both the primaryelectrically photosensitive ingredient and the primary colorant for saidparticle, and substantially simultaneously exposing said suspension toan image through said transparent electrode with a source ofelectromagnetic radiation whereby an image is formed.

2. The process as disclosed in claim 1 wherein said beta carotene ispresent in an amount from about 0.0025 to about 1.0 gram per gram ofsaid photosensitive pigment.

3. A method of photoelectrophoretic imaging comprising applying a layerof an imaging suspension to the surface of a first electrode, saidsuspension comprising a plurality of photoelectrophoretic imagingparticles in an insulating carrier liquid each of said particlescomprising an electrically photosensitive pigment which is both theprimary electrically photosensitive ingredient and the primary colorantfor said particle, said suspension further including beta carotene, saidfirst electrode being at least partially transparent to electromagneticradiation, contacting said suspension with a second electrode, applyingan electric field across said suspension between said electrodes andsubstantially simultaneously exposing said suspension to an imagethrough said transparent first electrode with a source of activatingelectromagnetic radiation whereby an image is formed.

4. The process as disclosed in claim 4 wherein said beta carotene ispresent in an amount ranging from about 0.0025 to about 1.0 gram pergram of said photosensitive pigment.

2. The process as disclosed in claim 1 wherein said beta carotene ispresent in an amount from about 0.0025 to about 1.0 gram per gram ofsaid photosensitive pigment.
 3. A method of photoelectrophoretic imagingcomprising applying a layer of an imaging suspension to the surface of afirst electrode, said suspension comprising a plurality ofphotoelectrophoretic imaging particles in an insulating carrier liquideach of said particles comprising an electrically photosensitive pigmentwhich is both the primary electrically photosensitive ingredient and theprimary colorant for said particle, said suspension further includingbeta carotene, said first electrode being at least partially transparentto electromagnetic radiation, contacting said suspension with a secondelectrode, applying an electric field across said suspension betweensaid electrodes and substantially simultaneously exposing saidsuspension to an image through said transparent first electrode with asource of activating electromagnetic radiation whereby an image isformed.
 4. The process as disclosed in claim 4 wherein said betacarotene is present in an amount ranging from about 0.0025 to about 1.0gram per gram of said photosensitive pigment.